how much is too much

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Old 02-20-04, 01:57 PM
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how much is too much

I need to know if it would be overkill to put two layers of polyiso sheathing on my basement wall for insulation, the equivalent of about R-20. Is this too much? I want it to be tight, but I don't want to overdo it if it's just a waste of time. Will I gain a lot by adding the extra layer? And is it alright in an application to place to sheets on top of each other like that?

thanks,
kevin
 
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Old 02-21-04, 11:23 AM
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If you think about it it is overkill the basment wall if its to the ground is not as cold as the weather that the upstairs wall gets. You dont say how you are doing this most put a V/B on the cement walls 2x4 studs with a R13 in it .A V/B over the studs and then the drywall or panel. no you dont want to put the sheets on top of each other if they have foil on them ED
 
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Old 02-23-04, 10:10 AM
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there is a vapor barrier on both sides of the insulation sheet, it is a kraft/foil/kraft triplex barrier, with the poyiso in between. i emailed http://www.pima.org for information, and they said that you could stack the insulation, but i wanted to check here too. are they wrong?

thanks,
kevin
 
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Old 02-23-04, 10:18 AM
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As I look at it Id say no dont do it. But if the company that made it said. Its ok to put one on top of the other do it.But get the name of who said to do it that way. You could do R13 batts for 1/2 the cost there I think.

I went to the www. there for your Polyiso board and I found what I though I cant see where you get that R-20. All of the polyiso that I looked at under any name came out to a R-6 for 1". so with two 1" you get a R 12. go with the batts ED
 

Last edited by Ed Imeduc; 02-23-04 at 11:09 AM.
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Old 02-24-04, 01:21 PM
MusicField
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Research done by Building Science Corporation (BSC) has shown that vapor barriers in basement insulation retrofits cause problems.

BSC recommends using extruded polystyrene insulation because it is a vapor retarder, not a vapor barrier.

Learn more by reading this:

http://www.buildingscience.com/resou...on_systems.pdf
 
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Old 02-26-04, 12:30 PM
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thanks for the info

i don't think moisture from the exterior is going to be a problem...the exterior walls are treated with a water proofing, and the contractor did a great job with the grading of the lawn - all the water that isn't diverted elsewhere by our downspouts flows very well away from the house. in addition, the concrete has had more than enough time to completely cure and dry out (the house was completed in 2000) before we apply anything to the walls. we've been there for three years, and the walls are very dry, have never seen even a little moisture, even when we've occasionally had very heavy rains ...

we are not using the batts or blanket insulation, rather we are using the rigid insulation that they mentioned...although the insulation does contain a vapor barrier on both sides.
 
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Old 02-26-04, 01:08 PM
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The reason why we have always put a V/B on the basement walls is not for the cold in the winter. Its for in the summer time when the basement feels cool and you let the warm moist air in the walls and floor will just sweat and sweat. Where did you get that R-20 for 2 sheets best I can do is R 12 for two 1" sheets ED
 
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Old 02-27-04, 06:22 AM
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my father-in-law is a roofing contractor - he purchases insulation through a rep with firestone building products. the insulation we are going to use is ISO-R, a polyisocyanurate rigid insulation that can be ordered anywhere from 1/2" up to 2" or maybe even thicker, i'm not really sure...at any rate, we are using 1-1/2" thick sheathing, that used to be rated with an R-value of 11.2 for that thickness...now they are rating it using LTTR, and it has an LTTR rating of 10.0 for the 1-1/2" thick sheathing. as I said in a previous post, they pay approx. $40 per 100 sq. ft. for it.
 
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